1. Field of the Invention
The present invention relates to a transmission device having an electric circuit. For example, a transmission circuit contained within a casing made of a synthetic resin for locking and unlocking a vehicle door locking member by remote control.
2. Description of the Prior Art
The transmission device of the aforementioned type bas been proposed and known, for example, in Japanese Laid Open Patent No. 288592,1990.
In this type of transmission device, it is necessary to minimize the size to make the device portable. Accordingly, a circuit board to which a transmission circuit is attached or incorporated is generally contained within a casing. Usually, this type of transmission device has a pushbutton, which is exposed on the surface of the casing, for transmitting a locking or unlocking signal. Further, the casing is often a combined structure comprising an upper case and a lower case during the assembling operation. Once assembled the circuit board is disposed near the switchcover and the joining faces of the casing.
When a user holds the transmission device in his hand as it contacts, or comes close to, an external ground, static charge accumulated in the body of the user tends to flow to the ground through the transmission device. Such an event might occur when the driver of a car holds a door open with the same hand used to hold the transmission device. A flow of charge, or a surge current, will occur if the voltage caused by the accumulated charge reaches a high enough level to break down all of the dielectric between the driver and the external ground. The dielectric between the driver's body and the external ground includes air gaps between conductive elements of the transmission device or solid dielectric materials such as portions of a plastic casing. The charge will flow, if the voltage is high enough, through the path of weakest dielectric strength. Points along such a path may include a thin-walled section of the switch cover, the joining face of the upper case and the lower case, or through a metal bolt used to secure a cover portion of the case. This path would also include the elements of the transmission circuit because of presence of interconnected conductive and semiconductive circuit elements. The possibility of such a breach and the consequent establishment of such a surge current in the circuit board can destroy IC chips or other circuit elements.